The measurement mechanisms for absolute pressure, relative pressure and differential pressure using hydraulic pressure transfer exhibit very narrow canals, in order to minimize the required liquid volume. This necessitates, however, a filling of the measurement mechanisms under vacuum. For standard applications, silicone oils having a low vapor pressure are used for the filling, in order that the work can be done at high vacuum without difficulty.
Measurement mechanisms for pressure sensors intended for use in oxygen plants must only be filled with fill liquids which have been specially permitted, in Germany by the Bundesanstalt für Materialprüfung BAM (Federal Institute for Material Testing), and are included in a special list under Unfallverhütungsvorschrift “Sauerstoff” (Accident Prevention Directive “Oxygen”) BVG B7 (VBG 62) of the Berufsgenossenschaft der chemischen Industrie (Professional Association of the Chemical Industry). Such permitted liquids are, for example, polychlorotrifluoroethylene, which is offered by various manufacturers under the tradenames or marks Halocarbon, Voltalef or Fluorolub. The O2-oils usable in the required temperature range from −20° C. encompass a mixture of molecules of different molecular lengths, with the partial pressure of the short-chained molecules lying in the region of 10−1 torr. If these fill oils are applied under usual vacuums, for instance <10−2 torr, then the short-chained components evaporate, which leads to the result that the composition can change to such an extent that the fill oils no longer satisfy the specifications of the oxygen permit. Additionally, the viscosity increases, whereby, on the one hand, the fill behavior is degraded, and, on the other hand, the lower limit of the temperature use range is raised. Thus, the problem concerns not only the fill oils for use in oxygen plants but also, in a certain way, all fill oils which involve a mixture of molecules of different molecular weights.
The effect of the evaporation of lighter components is, for example, evident on the basis of a mass spectrum, such as shown, for example, in FIG. 2. The continuous line shows the mass spectrum of the original fill oil, while the dashed line shows the spectrum of the fill oil, after having been held under a pressure which was too low.
It is, therefore, necessary to set the pressure in the filling installation such that an excessive outgassing of the fill oils is avoided. Concerning this, it was previously usual to let the pump for evacuating the fill chamber continue running with full power and to set the pressure in the fill chamber using a defined leak, for instance by way of a needle valve. This technique is disadvantageous in that ambient air enters continuously into the fill chamber to contaminate the surfaces of the fill chamber and the fill liquid e.g. with moisture in the air. Through this, the properties of the liquid can be degraded.